Separation of Zirconium and Hafnium by Solvent Extraction

Solvent extraction of zirconium and hafnium is one of the common methods for separating zirconium and hafnium. Compared with other zirconium and hafnium separation methods (such as pyrolysis separation and solvent extraction separation), this method has the advantages of large production capacity, simple process and easy to achieve continuously.

Principle

The extraction agents used for the separation of zirconium and hafnium mainly include ketone extractants, neutral phosphorus-containing extractants and amine extractants.

A commonly used ketone extractant is methyl isobutyl ketone (MIBK), which can form a neutral extract with hafnium thiocyanate and is preferentially extracted into the organic phase.

Methyl isobutyl ketone 3D ball
Methyl isobutyl ketone 3D ball. Source: Wikipedia

A typical neutral phosphorus-containing extractant is tributyl phosphate (TBP), which is preferentially extracted into the organic phase through the coordination of oxygen atoms in chemical bonds with zirconium metal atoms to form a neutral extract compound Zr(NO3)4•2TBP.

Ball and stick model of Tributyl phosphate
Ball and stick model of Tributyl phosphate. Source: Wikipedia

The commonly used amine extractant is trioctylamine (TOA). Trioctylamine forms an extract with zirconium ions in an acidic medium, and is preferentially extracted into the organic phase.

Process flow

There are three extraction processes: MIBK, TBP, and N235.

MIBK extraction

It uses ZrCI4 as raw material, adds water and NH4CNS ingredients. MIBK preferentially extracts hafnium, leaving a large amount of zirconium in the aqueous phase. This is the earliest extraction process used to separate zirconium and hafnium, and it is adopted by major producers of zirconium and hafnium such as the United States, France, Germany, and Japan.

TBP extraction

There are two aqueous feed systems for this process: nitric acid, and a mixed acid of nitric & hydrochloric acid. The former is to convert the product of zircon decomposed by alkali fusion method into nitric acid aqueous phase feed liquid, and use TBP to preferentially extract zirconium; the latter use Zr-CI4 as raw material, add water, nitric acid and hydrochloric acid as ingredients, and then use TBP to preferentially extract zirconium.

The separation coefficient of zirconium and hafnium in the TBP extraction process is large, and the number of extraction stages is small, and atomic-level zirconium oxide and hafnium oxide can be obtained at the same time. However, the water-phase feed liquid is highly corrosive, and the emulsification problem in the extraction process has not been completely solved, thus affecting its popularization and application.

N235 extraction

First, the zircon is decomposed by alkali fusion method, and the product is washed with water to remove silicon, and then leached with sulfuric acid to obtain a sulfuric acid solution of zirconium, and then the zirconium is preferentially extracted with N235. After washing, atomic-level zirconia containing hafnium <0.01% can be obtained. The hafnium in the raffinate is enriched to 50% to 70%, and then extracted by P204, and the zirconium and hafnium are further separated to obtain atomic energy level hafnium oxide containing more than 96% of hafnium.

This process has low material toxicity, light equipment corrosion, stable operation, and easy disposal of waste, so it is currently recognized as one of the best extraction processes.

Extraction equipment

There are two main types of extraction equipment, one is the extraction tower, and the other is the box-type mixer-settler. The former is used by the MIBK process, and the latter is used by TBP and N235 extraction process. The extraction tower occupies a small area and has a large production capacity. The box-type mixer-clarifier is simple in structure and stable in operation, and is generally made of acid-resistant materials such as plastic or plexiglass.

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3 Methods to Separate Zirconium & Hafnium

The two elements of zirconium and hafnium are symbiotic resources, which means that zirconium generally contains 0.5% to 2% of hafnium. However, the application of zirconium products in various industries requires high purity. For example, zirconium, which is a structural and cladding material for nuclear reactors, must contain less than 0.01% hafnium. In general, the metallurgical process of separating zirconium and hafnium is an important part of the zirconium metallurgical process.

The separation methods of zirconium and hafnium include pyrolysis separation, solvent extraction separation, and ion exchange separation. This article will briefly introduce these three separation methods.

Zirconium and Hafnium Pyrolysis Separation

Pyro separation is a method of separating zirconium and hafnium at high temperature or high pressure by using the difference in vapor pressure of zirconium and hafnium chloride. Zirconium and hafnium pyrolysis can replace the three production stages of extraction, calcination and chlorination in common separation methods. It has the characteristics of a short production process, high efficiency, low reagent cost and light pollution to the environment, and is a promising method for separating zirconium and hafnium.

The pyrolysis method is mainly realized by high-pressure rectification and molten salt rectification. High-pressure rectification is a process of directly separating zirconium and hafnium by using the difference in vapor pressure of Zrcl4 and HfCl4. Molten salt rectification is a process of separating zirconium and hafnium in a rectifying tower by using the difference in saturated vapor pressure of ZrCl4 and HfCl4 in KAlCl4 molten salt.

3 Methods to Separate Zirconium & Hafnium

Zirconium and Hafnium Solvent Extraction

This is a method for the separation of zirconium and hafnium using solvent leather. Compared with other separation methods of zirconium and hafnium, this method has the advantages of large production capacity, simple process, and easy to achieve continuously. It is the most important method for the separation of zirconium and hafnium.

The reagents used in this method mainly include ketone extractant, neutral phosphorus-containing extractant and amine extractant. There are three extraction processes of MIBK, TBP and N235. There are two main types of extraction equipment, one is the extraction tower, and the other is the box-type mixer-settler. The former is used by the MIBK process, and the latter is used by TBP and N235 extraction process. The extraction tower occupies a small area and has a large production capacity. The box-type mixer-clarifier is simple in structure and stable in operation, and is generally made of acid-resistant materials such as plastic or plexiglass.

Further Reading: Separation of Zirconium and Hafnium by Solvent Extraction

Zirconium and Hafnium Ion Exchange Separation

As the name suggests, this is a method for the separation of zirconium and hafnium by ion exchange. The production volume of this method is small. Only the former Soviet Union has used it to further separate zirconium and hafnium from the hafnium-rich material separated by the zirconium-hafnium recrystallization method to obtain hafnium oxide, which is used as the raw material for the production of atomic-level sponge hafnium.

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3 Types of Zirconium Based Getter Materials

Zirconium-based getter material refers to the alloy with high absorption active gas characteristics formed by adding other elements based on zirconium.

Before sealing in vacuum tubes and devices, the material must be heated and activated under vacuum conditions for fast and effective gettering. The function of activation is to remove the passivation film formed on the surface during the manufacturing and storage process to expose the fresh surface, which is conducive to the overall gettering, so as to achieve the purpose of absorbing a large amount of oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, hydrocarbons, and water vapor.

Zirconium-aluminum alloy, zirconium-graphite, and zirconium-vanadium-iron alloy are widely used zirconium-based getter materials today.

Zirconium-aluminum alloy getter

Zirconium-aluminum alloy getter can be made into ring-shaped material and composite strip-shaped material.

(1) Ring-shaped material. The material has poor gettering performance at room temperature and is usually not used for gettering at room temperature. This material is commonly used in electronic tubes, various vacuum devices, special lamps, inert gas purification, zirconium-aluminum getter pumps, etc.

(2) Composite strip material. The advantage is that the amount of mercury can be accurately controlled, and it does not decompose or generate mercury vapor below 500°C, thereby greatly reducing environmental pollution, preventing workers from mercury poisoning, and improving lamp quality and life. It has been widely used in fluorescent lamps and energy-saving lamps.

ZR1422 Zirconium Aluminum Alloy, ZrAl Alloy
ZR1422 Zirconium Aluminum Alloy, ZrAl Alloy

Zirconium graphite getter

Zirconium graphite getter is often used in high-reliability and long-life vacuum tubes and devices for long-term operation and storage, such as traveling wave tubes, X-ray tubes, trigger tubes, ceramic tubes, and laser tubes.

Zirconium-vanadium-iron alloy getter

Zirconium-vanadium-iron alloy getter is a low-temperature activated getter material composed of zirconium, vanadium, and a small amount of iron. It is divided into two types:

(1) Zirconium vanadium ferroalloy getter material, smelted by 70%zr+24.6%V+5.4%Fe in electric arc furnace or medium frequency induction furnace under vacuum or filled with inert gas, then crushed, pulverized, and then pressed into getter elements.

(2) (Zirconium vanadium ferro)/zirconium getter material. It is made by adding the zirconium vanadium ferroalloy powder prepared in (1), adding zirconium powder in a certain proportion, mixing evenly, and then pressing, high temperature and high vacuum sintering and other processes. into a suction element. Product forms are powder, flakes, rings, and strips.

These two zirconium-vanadium-iron alloy getters are low-temperature activated getters, and the activation process is as follows: the temperature is 400-600°C, the vacuum degree is 10-2-10-4Pa, and the maintenance is 10-30min. The working temperature is from room temperature to 350℃.

Zirconium-vanadium-iron alloy getter is widely used in stainless steel vacuum insulated cups (bottles), solar vacuum water heaters, high-efficiency oil-insulated pipes, and vacuum tube containers that are only allowed to operate at 500°C.

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Zirconium – A Vacuum Material

Properties of Zirconium

Zirconium easily absorbs hydrogen, nitrogen, and oxygen; zirconium has a strong affinity for oxygen, and oxygen dissolved in zirconium at 1000°C can significantly increase its volume. The surface of zirconium is easy to form an oxide film with luster, so its appearance is similar to that of steel. Zirconium is resistant to corrosion but is soluble in hydrofluoric acid and aqua regia. At high temperatures, zirconium can react with non-metallic elements and many metal elements to form solid solutions. Zirconium has good plasticity and is easy to be processed into plates, wires, etc. Zirconium can absorb a large amount of oxygen, hydrogen, nitrogen, and other gases when heated, and can be used as a hydrogen storage material. The corrosion resistance of zirconium is better than that of titanium, and it is close to niobium and tantalum. Zirconium and hafnium are two metals with similar chemical properties that are symbiotic together and contain radioactive substances.

Applications of Zirconium

Like lithium and titanium, zirconium can strongly absorb nitrogen, hydrogen, oxygen, and other gases. When the temperature exceeds 900 degrees Celsius, zirconium can absorb nitrogen violently; under the condition of 200 degrees Celsius, 100 grams of metal zirconium can absorb 817 liters of hydrogen, which is equivalent to more than 800,000 times that of iron. This characteristic of zirconium makes it widely used in the electric vacuum industry. People use zirconium powder to coat the surface of the anode and other heated parts of electric vacuum components and instruments to absorb residual gas in vacuum tubes. The high vacuum tubes and other electric vacuum instruments made in this way have high quality and long service life.

high vacuum tubes

Zirconium has a small thermal neutron capture cross-section and has outstanding nuclear properties, so it is an indispensable material for the development of the atomic energy industry and can be used as a reactor core structural material. Zirconium powder is easy to burn in the air and can be used as a detonator and smokeless powder. Zirconium can be used as an additive for deoxidation and desulfurization of high-quality steel and is also a component of armor steel, cannon steel, stainless steel, and heat-resistant steel.

Zirconium can also be used as a “vitamin” in the metallurgical industry to exert its powerful deoxidation, nitrogen removal, and sulfur removal effects. Adding 1/1000 zirconium to steel will increase the hardness and strength amazingly; zirconium-containing armored steel, stainless steel, and heat-resistant steel are important materials for the manufacture of defense weapons such as armored vehicles, tanks, cannons, and bulletproof panels. When zirconium is mixed into copper and drawn into copper wire, the conductivity is not weakened, while the melting point is greatly improved, which is very suitable for high-voltage wires. Zirconium-containing zinc-magnesium alloy is light and resistant to high temperatures, and its strength is twice that of ordinary magnesium alloys. It can be used in the manufacture of jet engine components.

Zirconium powder is characterized by a low ignition point and fast burning speed and can be used as a primer for detonating detonators, which can explode even underwater. Zirconium powder plus oxidant is like adding fuel to the fire, it burns with strong light and dazzling, and it is a good material for making tracer and flare.

Zirconium alloys and their applications

Zirconium alloy is a non-ferrous alloy composed of zirconium as the matrix and other elements are added. The main alloying elements are tin, niobium, iron, and so on. Zirconium alloy has good corrosion resistance, moderate mechanical properties, low atomic thermal neutron absorption cross-section in high temperature and high-pressure water and steam at 300-400 °C, and has good compatibility with nuclear fuel. In addition, zirconium alloy has excellent corrosion resistance to various acids, alkalis, and salts, and has a strong affinity with oxygen, nitrogen, and other gases, so it is also used in the manufacture of corrosion-resistant parts and pharmaceutical machinery parts. For example, it is widely used as a non-evaporable getter in the electric vacuum and light bulb industries.

zirconium alloy

There are two types of zirconium alloys produced on an industrial scale: the zirconium-tin series and the zirconium-niobium series. The former alloy grades are Zr-2 and Zr-4, and the typical representative of the latter is Zr-2.5Nb. In zirconium-tin alloys, the alloying elements tin, iron, chromium, and nickel can improve the strength, corrosion resistance, and thermal conductivity of the corrosion-resistant film, and reduce the sensitivity of the surface state to corrosion. Usually, Zr-2 alloys are used in boiling water reactors, and Zr-4 alloys are used in pressurized water reactors. In zirconium-niobium-based alloys, the corrosion resistance of the alloy is the best when the addition amount of niobium reaches the solid solution limit of the crystal structure of zirconium at the service temperature. Zirconium alloy has isomorphous transformation, the crystal structure is body-centered cubic at high temperature, and hexagonal close-packed at low temperature. Zirconium alloy has good plasticity and can be made into pipes, plates, bars and wires by plastic processing; its weldability is also good and can be used for welding.

Other Zirconium Compounds

Zirconium dioxide and zircon are the most valuable compounds in refractory materials. Zirconium dioxide is the main material of new ceramics and cannot be used as a heating material that resists high-temperature oxidation. Zirconium dioxide can be used as an additive for acid-resistant enamel and glass, which can significantly improve the elasticity, chemical stability, and heat resistance of glass. Zircon has a strong light reflection performance and good thermal stability and can be used as sunscreen in ceramics and glass. Zirconium can absorb a large amount of oxygen, hydrogen, ammonia, and other gases when heated, and is an ideal getter. For example, zirconium powder is used as a degassing agent in electronic tubes, and zirconium wire and zirconium sheets are used as grid supports and anode supports.

Powdered iron mixed with zirconium nitrate can be used as glitter powder. Zirconium metal is used almost exclusively as the cladding for uranium fuel elements in nuclear reactors. It is also used to make photographic flashes, as well as corrosion-resistant containers and pipes, especially hydrochloric and sulfuric acids. Zirconium chemicals can be used as crosslinking agents for polymers.

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Two Surface Treatment Technologies for Zirconium Materials

The surface of the zirconium rod and zirconium alloy must be clean and smooth before joining, heat treatment, electroplating and forming. This article introduces 2 types of surface treatment methods for zirconium materials.

  1. Surface decontamination

Grease, oil, and lubricants produced during zirconium machining or other processing can be removed in a number of ways. Commonly used cleaning methods are

1) cleaning with alkaline or milky detergent in a soaking tank;

2) cleaning with ultrasonic vibration;

3) rinsing with acetone or trichloroethylene or steam degreasing and

4) cleaning with other cleaning agents.

Small stains can also be removed by hand wiping with some solvents such as acetone, alcohol, trichloroethylene, or a trichloroethylene substitute. In the electrolyte system, if the voltage and current can be controlled to avoid anodic polarization or spark discharge and pitting, positive or negative polarity decontamination can be used. Before heat treatment and bonding, the surface of the zirconium material must be cleaned to prevent metal contamination and the resulting deterioration of ductility.

Surface Treatment Technologies for Zirconium Materials

  1. Blast cleaning

Mechanical decontamination methods such as sandblasting, shot blasting, and evaporative cleaning can remove dirt and lubricants from zirconium and hafnium surfaces. Alumina, silicon carbide, silica and steel grit are ideal media for mechanical decontamination. The decontamination medium used should be replaced regularly to avoid increased workload due to particle passivation.

Grinding or shot peening may cause residual compressive stress and thermal deformation on the surface of the material, especially the surface of the sheet. Hot deformation may also occur during subsequent rolling and profile machining.

Blast cleaning is not a substitute for pickling. Blast cleaning cannot remove surfaces contaminated with interstitial elements such as carbon, oxygen, and nitrogen. In general, blast cleaning followed by pickling can ensure the complete removal of surface contamination and cold-worked layers, resulting in a smooth, shiny metal surface.

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How to Ensure the Welding Quality of Zirconium Alloy

In the previous article, we introduced the basic properties of zirconium alloys and the problems that easily occur during the welding process. Next, we will explain how to ensure the welding quality of zirconium alloys and some precautions.

Precautions for zirconium alloy welding

  • In the welding prefabrication stage of a large number of welds, a special closed clean place needs to be set up, and strict control of environmental dust pollution and air humidity. For example, when entering the construction site, measures such as wearing clean labor insurance shoes must be worn to ensure the cleanness of the welding environment. In the outdoor installation environment, make a temporary operating room to achieve clean conditions.
  • Strengthening the requirements for the weld joint groove and within 70mm of both sides of the groove and the cleanliness of the surface of the welding wire is an important factor to ensure the welding quality.
  • In the welding process of zirconium alloy, pores are the most prone to defects, and it is mostly concentrated near the fusion line and the centerline of the weld. The most critical steps to prevent the occurrence of welding porosity defects are to strengthen the control of the cleanliness and humidity of the welding environment, and to enhance the cleaning of the bevel and the surface of the welding material, so as to improve the quality of the internal and external protection of high purity argon in the weld zone.
  • The zirconium alloy has a low thermal expansion coefficient, a small amount of thermal deformation, and a small volume change during phase change. It has a low content of impurities such as sulfur, phosphorus, and carbon, so there is no obvious tendency to form cracks during welding. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the weld in the peer group, a cold crack will occur. In addition, the hydrogen atoms have the property of diffusing and accumulating to the high-stress parts in the heat-affected zone at a relatively low temperature, which promotes the formation of relatively weak links in these parts, which may lead to the occurrence of delayed welding cracks.
  • In the welding test, manual tungsten argon arc welding with low welding line energy and convenient gas welding protection should be selected; The larger-diameter welding torch nozzle, the outer surface of the weld seam, and the internal argon filling method of the pipe are used for air isolation to achieve the purpose of the weld seam not being oxidized and absorbing harmful gases.
  • The filler wire used for zirconium alloy welding should be selected according to the principle of matching the composition of the base metal. The surface of the welding wire must be free from defects such as heavy skin, cracks, oxidation, and metal or non-metallic inclusions. The welding wire should be cleaned and dried before use.
  • Zirconium alloy tungsten arc welding requires high-purity argon with a purity of not less than 99.999%, and its impurity content meets the requirements of the current GB / T4842 standard. Due to the extremely high requirements for the purity of the welding protective gas, the welding process needs to be continuously inflated and cannot be interrupted halfway, otherwise, the argon filling must be replaced again. The method of using an ordinary single bottle of argon direct gas supply cannot meet the protection requirements. Multiple bottles of argon gas need to be connected in series to increase the gas supply capacity, and multiple welders can be operated simultaneously by dividing the cylinder.
  • Because zirconium alloys are active at high temperatures, relying solely on the argon gas supplied by the argon arc welding torch nozzle to protect the molten pool and high-temperature bead and heat-affected zone during welding cannot guarantee the welding quality. In order to ensure that the requirements for gas isolation in high-temperature areas and prolonged argon protection time are met, special external gas protection devices for pipes must be added to provide high-purity argon isolation protection for weld pools, high-temperature weld beads and heat-affected zones at high temperatures.

Stanford Advanced Materials supplies high-quality zirconium alloy products to meet our customers’ R&D and production needs. Please visit https://www.samaterials.com/ for more information.

How Did Nuclear Zirconium Alloys Develop?

Zirconium alloys have a small thermal neutron capture cross-section (0.185b) and are surprisingly resistant to corrosion, so they are widely used in fission reactors, such as core-clad tubes, grids, and guide tubes in boiling water reactors, as well as pressure pipes and exhaust reactor vessels in pressurized water reactors.

Nuclear zirconium alloy

With the application of zirconium alloys in the nuclear energy industry, the zirconium industry has developed rapidly.

In the nuclear giant change reactor, nuclear fuel is fission reaction all the time. In the reaction, the neutron bombards the nucleus of U235, which splits into Ba140 and Kr93, and releases two or three neutrons at the same time; other U235 nuclei are bombarded by these neutrons and re-fission. This is the chain reaction of fission.

nuclear-reactor

A material with a large neutron capture cross-section will absorb many neutrons when they hit the wall, reducing the efficiency of the chain reaction. Meanwhile, the chain reaction produces a lot of heat, which is removed by circulating cooling water (or other coolants) to avoid overheating and damage to the reactor. When metals come into contact with high-temperature water, they can be corroded (oxidized). Materials with poor corrosion resistance need to be replaced frequently, which increases the cost and easily leads to safety accidents. Therefore, as core-cladding and structural materials, zirconium alloys are required to have low neutron capture cross-section and excellent corrosion resistance, so the development of zirconium alloys should be attributed to the nuclear industry.

Origin of zirconium alloys

Initially, zirconium was not considered a suitable material for use in the nuclear industry, because studies have shown that zirconium’s effect on thermal neutron absorption can affect the efficiency of nuclear reactors. Later, researchers at the Oak Ridge Institute found that 2.5% of the hafnium in zirconium was responsible for its large thermal neutron capture cross-section.

zirconium alloy

Zirconium and hafnium are associated with ore and are generally difficult to separate. Until the 1850s, Admiral in the Naval Nuclear Propulsion project decided to use zirconium in the water-cooled reactor of the Nautilus Nuclear Submarine. Although zirconium had already been used for the project by that time, there were no strict standards for the use of zirconium, and the researchers only knew that improving the purity of zirconium would be good for the properties of the alloy. Some processes are used to purify strip zirconium, but it still contains small amounts of nitrogen, making it less resistant to corrosion at high temperatures. Finally, the researchers realized that purity was not the key to zirconium’s corrosion resistance, because they found that some zirconium materials containing impurities (such as tin, iron, chromium, and nickel) were more resistant to corrosion than higher-purity zirconium materials. Therefore, the development of zirconium alloys is put on the agenda.

Development of zirconium alloys

The first alloy, Zircaloy-1, contains 2.5% tin. It was found that the corrosion rate of Zircaloy-1 alloy was increasing and not consistent with the expected decrease. This was similar to a normal sponge zirconium material, so Zircaloy-1 was quickly abandoned.

At the same time, the researchers found that adding iron and nickel to the Zircaloy-2 could improve corrosion resistance. The tin content was reduced to 1.5% and 0.15% iron, 0.05% nickel and 0.10% chromium were added. It was found that Zircaloy-2 had the same mechanical properties as Zircaloy-1, but the high-temperature corrosion resistance of Zircaloy-2 was much better than that of Zircaloy-1. However, during the service of the pressurized water reactor, the alloy produces a lot of hydrides, resulting in hydrogen embrittlement.

By studying the binding technique, the researchers found that nickel greatly enhanced the hydrogen absorption capacity of zirconium alloys. The researchers removed the nickel from the Zircaloy-2, creating a Zircaloy-3. But Zircaloy 3 was quickly abandoned because its strength was too low. In addition, Zircaloy-3 produced many striated Fe-Cr binary intermetallic compounds when it was processed in the two-phase zone, so it could not provide sufficient corrosion resistance. The strength of Zircaloy-3 was still too low, although changes in the heat treatment process prevented the production of the striated compound.

The researchers compensated for the nickel by increasing the iron content by 0.22 percent and found that the corrosion resistance of the new alloy was similar to that of zircaloy-2, which had only half the hydrogen absorption rate. The new alloy quickly became a major part of the pressurized water reactor, the first Zircaloy-4.

Zirconium alloys for the nuclear industry have been developed into the third generation of products, which are used in various reactors.

The first generation is the standard zircaloy-4 and Zircaloy-2, whose composition and process requirements are specified in the ASTM standard. This generation of zirconium alloy is still in use.

The second generation is low tin Zircaloy-4 and optimized Zircaloy-4. The tin content of low tin Zircaloy 4 decreased from 1.2% ~ 1.70% to 1.20% ~ 1.50%, and the carbon and silicon were controlled at 0.008% ~ 0.020% and 0.005% ~ 0.012%, and the cumulative annealing process parameters in the alpha phase after quenching in the beta phase were strictly controlled; the optimized zircaloy-4 is based on the low tin zircaloy-4, and the content of alloy elements and process parameters are more strictly controlled, so as to improve the uniformity of materials.

The third generation of zirconium alloy has excellent properties and is widely used as a fuel rod cladding tube and fuel assembly guide tube. NDA and MDA from Japan, HANA from South Korea, and composite casings from Siemens are also examples of this generation of products.

Prospect of zirconium alloys

Zirconium alloys above 620℃ (depending on composition) convert to body-centered cubic β-zirconium. After the transformation, the mechanical properties and corrosion resistance of the alloy will be greatly reduced, and it cannot continue to maintain the safe operation of the nuclear reactor. The famous event is the accident at the Fukushima nuclear power plant in Japan. Affected by the big earthquake in eastern Japan, the reaction water of the Fukushima nuclear power plant leaked, and the cladding temperature increased significantly. The zirconium alloy cladding softened quickly, and brittle material formed with the leakage of air, leading to the leakage of nuclear fuel. Large amounts of nuclear-contaminated water flowing into the sea have caused great damage to the ecology of the world.

As a nuclear reactor cladding material, it needs to have a small thermal neutron capture cross-section, which leads to the zirconium alloy cannot be highly alloyed, so it is bound to be difficult to break through the zirconium alloy’s high-temperature performance. At present, countries attach great importance to this problem. On the one hand, they are trying their best to make a breakthrough in the high-temperature performance of zirconium alloy; on the other hand, they are looking for alternative products of existing fuel cladding, such as silicon carbide (SiC) composite material, molybdenum alloy, cobalt alloy and so on. Molybdenum alloys and cobalt alloys were originally intended as structural materials for fusion reactors. Although they do not have the same low thermal neutron absorption cross-section as zirconium alloys, they have excellent high-temperature stability.

Stanford Advanced Materials supplies high-quality zirconium alloys to meet our customers’ R&D and production needs. Please visit http://www.samaterials.com for more information.

What Do You Know About Zirconium And Zirconium Alloys?

Compared with traditional iron, copper, nickel, and other metal elements, zirconium has a lower density and smaller thermal expansion coefficient. In addition, zirconium has a low thermal neutron absorption cross-section (only 0.18×10-28 m2) and good corrosion resistance, which makes zirconium and zirconium alloys have a wide range of applications in the nuclear industry, aerospace and other special fields.

Zirconium and its alloys have been widely used as cladding materials in nuclear reactors. Zirconium and its alloys reflect neutrons back into the reactor more efficiently than stainless steel, greatly saving uranium fuel; Zirconium alloy has good corrosion resistance at high temperature and high-pressure steam of 300 ~ 400 ℃, which also makes the reactor have a long service life. Therefore, zirconium is regarded as the first metal in the atomic age.

zirconium alloy

Development status of zirconium and its alloys

Zirconium, which is found in the earth’s crust at about 220 g /t, ranks 20th, ahead of other common metals such as copper, nickel, lead, and cobalt. Initially, zirconium alloys were mainly used as cladding materials in the nuclear industry. In recent decades, zirconium alloys have been widely used in the chemical industry, medical industry, and some special fields.

Zirconium alloy for nuclear use

Zirconium alloys have been widely used in the nuclear industry because of their very low thermal neutron absorption cross-section and good resistance to high temperature and pressure corrosion. France, the United States, Germany, and Russia have developed a series of zirconium alloys for nuclear use. At present, Zr-2, Zr-4, Zr2.5nb and ZIRLO, E635, M5, and NDA zirconium alloys have been successfully applied in the nuclear industry. These newly developed zirconium alloys have lower radiation creep properties and better resistance to iodine stress corrosion. In addition, they are able to meet the requirements of high burnup of the fuel assemblies, increasing the service life of the assemblies to 30 years.

nuclear reactor 2

Corrosion-resistant zirconium alloy

Zirconium has excellent corrosion resistance against most organic acids, inorganic acids, strong alkalis, and some molten salts. Therefore, zirconium can be used to improve the service life of some key components in corrosive environments. Another way to improve the corrosion resistance of alloy parts is surface pretreatment. In industry, zirconium is placed in high-temperature air to obtain a dense oxide film, so as to improve the corrosion resistance and erosion resistance of zirconium and its alloys. The results show that the corrosion rate of zirconium treated by surface oxidation in sulfuric acid medium is only 5% of that of pure zirconium, but the erosion resistance is increased by twice.

At present, zirconium is widely used as corrosion-resistant material in the chemical industry, and it has been widely used in the heat exchanger, dike washing tower, reactor, pump, valve, and corrosion medium pipeline. For example, zirconium alloys have been used to produce concentrated and hydrolyzed tubes in hydrogen peroxide production lines, while zirconium pressure reducing valves, agitators and flow meters are used in fertilizer production, sewage treatment, and dye industries.

Biomedical materials are a new high-tech material in recent years, and biomedical alloys must have good compatibility and corrosion resistance with the environment of biological fluids. Zirconium is valued by researchers for its good biocompatibility, elastic modulus similar to bone and corrosion resistance. Ti6Al4V, a titanium alloy implanted earlier in hard tissues of the human body, has an elastic modulus of nearly 110 GPa, which is much higher than the elastic modulus of 15 ~ 30 GPa of natural bones of the human body.

High-strength zirconium alloy

In the fields of space exploration, deep-sea exploration, and high-speed railway, there are often some special operating environments, such as the alternating temperature environment of -200 ~ 200 ℃, continuous space irradiation, and relative motion of structural parts, etc. Under these special circumstances, long-serving structural components are often faced with fatigue damage, dimensional instability, atomic oxygen erosion, and friction wear. At present, the structural parts used in these special fields are mainly made of 20Cr, GCr15, and other alloy steel materials, which often have problems such as poor radiation resistance, easy damage of moving parts, high density, and high cost.

Compared with traditional alloy steels, zirconium and its alloys have several important potentials:

  • The thermal expansion coefficient of zirconium is small and the size structure is stable, so it has the potential to produce precise structural components;
  • It has the potential of resisting space radiation damage;
  • It has the potential to resist atomic oxygen erosion;

Therefore, zirconium and its alloys are expected to adapt to unconventional environmental conditions in special fields and have the potential to be used as structural components in special environments.

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Argon Arc Welding Technology of Zirconium and Zirconium Alloy

Zirconium and zirconium alloys have excellent corrosion resistance to acid and alkali, and even surpass niobium, titanium and other metals in some media. Therefore, zirconium and zirconium alloys are gradually used as structural materials such as equipment and pipelines in the chemical industry with strong corrosion resistance due to their good corrosion resistance in recent years.

Due to the high-temperature chemical activity, zirconium and zirconium alloys can react with various elements in the air at high temperature, thus damaging their mechanical properties. Therefore, in the process of zirconium and zirconium alloy welding, the key to ensuring the quality of welding is to select a clean operating environment and strengthen the isolation and protection of welding seams and parts in the heat-affected zone.

Basic properties of zirconium and zirconium alloys

Zirconium and zirconium alloy materials mainly include R60702, R60704, and R60705. Zirconium and zirconium alloys have good welding properties and stable chemical properties at room temperature. However, its high-temperature chemical properties are very active, and it has a strong affinity for the pollution of oxygen, nitrogen, hydrogen and dust and humidity in the operating environment.

The excellent corrosion resistance of zirconium and zirconium alloys comes from the oxide film formed on the surface and depends on the integrity and firmness of the oxide film. When zirconium and zirconium alloy absorb a certain amount of oxygen, nitrogen, hydrogen, and other gas impurities, their mechanical properties and corrosion resistance will decrease sharply. Therefore, strengthening the protection of the surface of environmental dust, humidity and heat affected area and the back of the welding seam is the key element of quality control in the welding process.

Zirconium and Zirconium Alloy

Factors influencing the welding quality of zirconium and zirconium alloy

  • The tendency of weld cracks

Due to the low thermal expansion coefficient of zirconium and zirconium alloy, the volume change caused by thermal deformation and phase change is very small, and the content of sulfur, phosphorus, carbon and other impurities is very low, there is no obvious trend of cracks in the welding process. However, when the welding seam absorbs a certain amount of oxygen, nitrogen and hydrogen gas impurities, the performance of the welding seam and the heat-affected zone will become brittle. If there is stress in the welding seam, cold cracks will occur.

At the same time, hydrogen atoms have the property of diffusing and aggregating to the high-stress parts in the heat-affected zone with lower temperature, which leads to the formation of relatively weak links in these parts, which may lead to the generation of welding delay cracks.

  • Selection of welding materials

The filler wire for zirconium and zirconium alloy welding should be selected according to the principle of matching the base material composition. The surface of welding wire shall not have heavy skin, crack, the oxidation phenomenon and metal or non-metal inclusion defects. Besides, the welding wire should be cleaned and dried before use.

  • Selection of protective gas

Argon arc welding with tungsten electrode of zirconium and zirconium alloy shall adopt high purity argon with 99.999% purity and the impurity content shall meet the requirements of GB/T4842 current standards.

Due to the extremely high requirements on the purity of welding protective gas, continuous gas charging is required during the welding process, and the gas cannot be interrupted in the process; otherwise, argon charging needs to be replaced again. Therefore, the direct gas supply method using ordinary argon in a single bottle cannot meet the protection requirements. It is necessary to increase the gas supply capacity of multiple argon bottles in series and satisfy the simultaneous operation of multiple welders through the air separation cylinder.

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